In communication between servers in a data center and optical transmission systems constituting a core network, an optical transceiver to perform mutual conversion between an electric signal and an optical signal and to perform transmission and reception of the optical signal is heavily used in a physical layer of an OSI reference model. The optical transceiver includes a transmission part (optical transmitter) and a reception part (optical receiver). The optical transmitter includes a light source such as a semiconductor laser, an optical modulator to modulate CW (Continuous Wave) light output from the light source in response to a driving voltage and emit modulated CW light (optical signal), and an optical modulator driver to generate the driving voltage according to an electric signal and provide a bias voltage to be a reference voltage thereof. The optical transmitter converts the electric signal into the optical signal and transmit the optical signal to an optical waveguide such as an optical fiber (this corresponds to the case of an external modulation system (refer to FIG. 1)).
In order to drive the optical modulator at a symbol rate of 25 [Gbaud] or 40 [Gbaud], for example, a superior quality of waveforms such as high-speed falling/rising and low jitter is required in waveforms of driving signals. For this reason, it is necessary to suppress a reflection coefficient of a high frequency signal small up to a frequency of about 40 [GHz] for example. Patent Literature 1-3 disclosed driving techniques to suppress an S parameter S22 of a driver and an S parameter S11 of the optical modulator.
Patent Literature 1 disclosed technology for applying a technique of a travelling wave amplifier (TWA) to a differential amplifier. The TWA includes transmission lines formed between input terminals of a plurality of transistors and between output terminals thereof to broaden bandwidth. Patent Literature 2 disclosed a method of improving a frequency characteristic in a high frequency region when a TWA of a differential type (semiconductor IC chip) is mounted on a package (not illustrated in the drawings). Patent Literature 3 disclosed a capacitive source degeneration circuit as technology for improving a frequency characteristic having a right-downward slope caused by signal attenuation in an input transmission line (not illustrated in the drawings).
Digital coherent optical transmission systems having a large capacity of communication and a high speed performance have been developed for practical use to deal with a rapidly increasing demand of communication traffic for a network. Such digital coherent optical transmission systems have been incorporating PAM-4 (4-level Pulse Amplitude Modulation) and 16-QAM (16-level Quadrature Amplitude Modulation) as a modulation system.
Patent Literature 1: Japanese Patent Application Laid-Open No. H09-130170,
Patent Literature 2: Japanese Patent No. 4965602,
Patent Literature 3: Japanese Patent No. 4952713.